Strabismus is the term used to describe an anomaly of ocular alignment. Strabismus can occur in one or both eyes, and in any direction. The terms used to describe strabismus depend upon the direction of deviation, the conditions under which it is present, and whether it changes with the position of gaze.

Nasal deviation (relative to the fixating eye) is described with the prefix "eso", and temporal deviation with the prefix "exo". As a general rule, the prefix "hyper" is applied to the eye that is more superior in vertical deviations, regardless of which eye is fixating. However, the prefix "hypo" is sometimes used to denote an eye that is depressed relative to the fixing eye, usually when there is a restrictive hypotropia.

A "latent" strabismus is present only when fixation is interrupted and is known as a "phoria" (eg, esophoria, exophoria). In children with latent strabismus, ocular alignment is maintained by fusion as long as fixation is uninterrupted. Strabismus that is present without interruption of the visual axis is "manifest" and described as a "tropia".

Manifest strabismus can be intermittent, occurring only when fusional capabilities are exceeded (eg, when the child is tired) or constant [1].

Manifest strabismus can be monocular, when deviation always involves the same eye, or alternating, when either eye may deviate.

Deviation that is the same in all positions of gaze is described as "comitant". Deviation that changes depending upon the position of gaze is "incomitant" and usually is present with paralytic or restrictive strabismus.

Unsteady ocular alignment is often present in normal newborns during the first few months of life [2] and can be described as "ocular instability of infancy".

NEUROANATOMY

Three pairs of extraocular muscles move each eye in three directions from the primary position: vertically (superior and inferior), horizontally (medial and lateral, or adduction and abduction), and torsionally (intorsion when the top of the eye rotates nasally and extorsion when the top of the eye rotates temporally) (figure 1). The following muscles are responsible for these movements (table 1):

The superior rectus and inferior oblique muscles are both responsible for upward vertical movements

The inferior rectus and superior oblique muscles are both responsible for downward vertical movement

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